Compression Question

[bigreddaddy]

Pulled the plug out, put my finger on the hole, turned it over and have compression. I hooked up a compression tester, turn it over and it doesn't even show a reading. There was enough force to almost move my finger off the hole. So either my tester somehow broke itself while in the tool box or maybe its a stuck valve??? The motor is a Yammy 350cc 4 stroke.
The guy I got it from said he was hauling azz down the road and went to gun it and the motor lost compression. Didn't smoke, no loud noises, just died. The motor turns over fine and there isn't any odd noises. No oil leaks.
Stuck or broken valve, or maybe blown head gasket?

[NINJA]

Timing chain jumped a tooth or two?

[bigreddaddy]

I just don't understand why the tester is not getting any reading when I can easily feel a good amount of air coming out the plug hole.

[longbedGTs]

Have you tried the comp. tester on another machine? Verify the tester is good first.

[Dirtcrasher]

22 years ago I met a great mechanic. He was much older than me but liked me because I loved to clean the parts and watch him fix stuff. One thing he told me about compression was "does it blow your finger off" I'm sure he had a gauge but thats what he told me to do to check compression. Does it fart? or blow your finger off?........

[bigreddaddy]

Not quite enough to blow my finger off the hole. I'm having to jump the battery to turn it over because the battery is shot. So it doesn't have the juice it should, but it is plenty to turn the motor over. I know my tester works, BUT yes I will use it on another machine just to eliminate it a problem.

[RedRider_AK]

If it doesn't blow your finger off, it's not enough compression. I had an old 110 engine that you could literally pull over with 2 fingers, with a plug in it, and it still had enough compression to pop my finger off the hole pretty good.

I doubt it's a head gasket. Check around the head and see if there's any leaking of anything around it. Might be a timing problem or a burnt valve, or one that's not closing all the way... Maybe even bent. Who knows?

[bigreddaddy]

I think one problem is the jump starter I'm using has a battery, and it was real low. So I'm charging the jumper so I can jump the stupid battery.
Anyway, I'll take another reading when its fully charged.

By the way the 250es worked GREAT tonight pulling my 5x10 heavy duty trailer and a bunch of kids and adults around the neighborhood to trick-or-treat. There were 2 other guys w/quads doing the same thing but I got all the looks! lol

[bigreddaddy]

Compression tester checks out. I tested my 250es and have 150psi. I adjusted the valves in the 350cc and still nothing on the tester. Looks like its going to get a new top end.